Electronically rebalanced temperature sensing bridge



Jan. 3, 1956 c. E. IMPEY ET A1. 2,729,396

ELECTRONICALLY RERALANCED TEMPERATURE sENsING BRIDGE ma. Nn

.51 New GNN wN/V am.

Nrn

ww@ my 5mn@ w Dh mfcwa W Q www ,w E QQ m2 l E im., N. IWB N 1+ WR.. @mlbW um @MTHw www mw mw ER mm wm E sum um 1hr MT* E@ Sou k Mmb v ME scm `wlm EN ma E.. E mm United States Patent O ELECTRONICALLY REBALANCEDTEMPERA- TURE SENSING BRIDGE Charles E. Impey, Brookfield, and Joseph A.Christ and Edward W. Johnson, Chicago, Ill., assignors to Vapor HeatingCorporation, Chicago, Ill., a corporation of Delaware t ApplicationSeptember 12, 1951, Serial No. 246,188 14 Claims. (Cl. 236-68) Thisinvention relates to a temperature control system in which an electricalbridge, utilized as a temperature sensing element, becomes unbalancedunder the influence of deviations in temperature from a desiredcondition.

A principal object of the invention s to provide, in a temperaturecontrol system of the above character, im-

provements whereby the bridge, being unbalanced in response to adellection in temperature from a pre-selected value, may be recurrentlyrebalanced during the continuance of said temperature deflection.

Another object of the invention is to provide improved means adapted tofunction, when the bridge is unbali ance the same and whereby thiscycling of the bridge is utilized to effect a series of adjustments to ameans for correcting said temperature deflection and thereby rebalancethe bridge. i

According to the present invention, a temperature sensing bridge,provided in one or more of its arms with resistance elements which arehighly sensitive to temperature changes, is energized by a suitablesource of electrical energy and is so adjusted that all arms of thebridge are in perfect electrical balance when a desired temperaturecondition is maintained. In the event of a temperature deviation of apredetermined value away from the desired temperaturecondition, thebridge becomes unbalanced and this unbalanced condition is utilized toactivate electronically controlled means forlrebalancing the bridge.

The present invention provides means whereby the bridge may berebalanced'in either of two ways'. For example, by producing, in anotherportion of the bridge, a resistance variation corresponding in valuetothe re` sistance variation produced by the existing temperaturedeflection or by soy altering the unbalancing temperature influence onthe bridge as to remove the temperature deflection. in order torebalance the bridge by producing resistance variation therein to offsetthe resistance-due to the existing temperature deection,` electrical`heaters are associated with selected portions of the bridge. VWhen thebridge is unbalanced by a temperature deflection which varies theresistance of one portion of `the bridge relative to another portionthereof, the bridge may be rebalanced by altering the temperature of anopposing portion of the bridge to produce a corresponding resistancevariation therein. When the bridge is rebalanced by correcting thetemperature deflection affecting the bridge, this correction is obtainedby appropriately vary-v ing the effectiveness of the ntemperaturealtering device so as to increase or decreaseeits heatingor coolingfunc-V tion, as may ,be required to remove said temperature deflection.

A transformer comprising a primary and three secondary coils, woundabout a common core, is utilized as a power source for the bridge andfor the electronic control elements of the system. A junction of thearms forming one branch of the bridge is grounded and the opposite sidesof the bridge are connected to opposite ends of one secondary winding ofthe transformer. Another secondary coil of the transformer is connectedby a center Vtap to ground and has its opposite ends connected throughseparate control relays with an output point of the bridge. Anelectronic amplifier, interposed in the connection between the outputpoint of the bridge and the said relays, includes a voltage amplifierand a pair of electronic tubes which, in addition to functioning aspower amplifiers also function as phase selectors or switches toselectively control the energizationV of the relays in relation toincreases and decreases in the resistance of portions of the bridge. Theselective control of the said relay is obtained by virtue of theconnection of one relay and its associated phase selector tube with oneend of its associated secondary winding of the transformer, whereby itwill function in response to an unbalanced condition ofthe bridgeproduced by a decrease in resistance of one side thereof.

The other relay and its associated phase selector tuberare connected tothe other end of said secondary winding and thereby respond to anunbalanced condition of the bridge resulting from an increasedresistance in the last mentioned side of the bridge.

When the bridge is unbalanced a voltage appears at the output pointthereof and the voltage is impressed on a grid element of an electronicvoltage amplifier. The signal thus amplified is directed through anelectronic power amplifier and thence to energize a selected relay whichfunctions to close an energizing circuit through one of said electricalheater and thereby rebalance the bridge. However, when the bridge isrebalanced by the energization of an electrical heater, such rebalancedcondition is maintained only momentarily, since the rebalancing of thebridge deactivates said electronic amplifier and therefore deenergizesthe relay for energizing the said electrical heater. This momentaryrebalancing action reoccurs intermittently during the continuance of thetemperature deflection tending to bias the bridge to an unbalancedcondition and therefore creates a so-called cycling action of thebridge. The cycling of the bridge produces intermittent activation ofthe electronic amplilier and the relays controlled thereby and thisintermittentaction is utilized to modulate the effectiveness of atemperature altering device so as to restore the ambient temperatureeffect on the bridge to the normal control level and thereby rebalancethe bridge.

In addition to the provision of one or more temperature `sensitiveresistors adapted to respond to temperature deflections within anenclosed space orV zone whose temperature is to be controlled, theimproved system includes other temperature sensitive resistors, forexample, ar pair of such resistors interposed in the bridge andpositioned to respond `to abrupt changes in the temperature of theheating and cooling media delivered into the enclosure. In thisconnection one of the said resistors is partially shielded from thetemperature of said media so as to lagA in its response to said abrupttemperature changes. The lag effect thus produced creates yan unbalancedcondition in the bridge and therefore effects an adjustment of thetemperature altering device in a direction to correct the said abrupttemperature variation.

In order to avoid activating the system in response to minor deviationsin temperature, means are providedwhereby the bridge is renderedineffective to activate theelectronic amplifier until the temperaturedeviation is 'ofa predetermined value, that 's to say, a valuecorresponding to the upper or lower limits of a pre-selected temperaturecontrol band. The invention also includes adjustable means whereby thespacing between the upper and lower limits of the temperature controlband may be varied and other means whereby the temperature band as awhole ymay be raised or lowered as may b'e desired.

A preferred embodiment of our improved control system is illustrated inthe accompanying drawing as comprising a temperature altering device,designated generally by the reference letter A and adapted to heat orcool an enclosure B; a temperature sensing bridge C whose resistance`becomes unbalanced by the effect of temperature changes 'on localizedportions thereof; a pair of relays designated D, D' for closingenergizing circuits through an Vactuator E associated with saidtemperature altering device A, and for closing energizing circuitsthrough a pair of electrical 'heaters F, F associated with selectedparts of the bridge C, .and electronic means, designated as a whole bythe reference letter G, connected in the circuit between the bridge Aand the relays D, D' and functioning to selectively control theenergization of said relays Vin response to unbalanced conditions of thebridge resulting from increases and decreases lin the resistance ofopposing parts of the bridge.

Temperature altering device The temperature altering device A, for thepurpose of convenience of illustration, is shown diagrammatically hereinas an air blending device comprising a conduit divided by a partition1.1 into a `hot air duct 12 and a cool air duct 13. A damper 14 ispivotally supported at one end and is illustrated in a position wherebyequal amounts of heated and unheated air are blended and deliveredthrough an outlet 15 into the enclosed space B whose temperature isbeing controlled. It should be understood, however, that various othertemperature altering devices may be used, the type of such device beingdetermined largely by engineering factors and character of theenclosure, thing or surface whose temperature is -to be -controlled.

Damper actuator The damper 14 is operatively connected by means of vconventional driving connections 17 and l18 to the actua- .tor E, forexample a reversible electric motor. The motor is adapted when rotatedin one direction to move the damper 14 in a direction to close orpartially close the cool air duct 13 and thereby proportionatelyincrease the eiectiveness of the heated air delivered. When the motor Erotates in a reverse direction, the damper 14 lis moved in a directionto close or partially close the hot air duct 12 and therebyproportionately increases the cooling eiect of the unheated air beingblended with the heated air. The energizing circuit for operating themotor E in the direction to reduce the heating effect leads from apositive 'line 19 through a de-energized closed contact 20 of relay D,conductor 21 to energized closed contact 22 of relay D' and thencethrough the conductor 23 and motor ield winding 24 and limit switch 25through the armature of .the motor E and thence to the ground 26. Theenergizing circuit for reversing the direction of the motor E to operatethe damper 14 in a direction to increase the vheating and reduce thecooling effect of the temperature altering device A, leads from the saidpositive line 19 through the energized closed contact 20 of relay D,conductor 27, motor field winding 24a, closed limit switch 28 and thencethrough the motor armature E to the ground 26.

The limit switches 25 and 28 are shown diagrammatically, but it will beunderstood that they are moved in a well-known conventional manner totheir open positions by the movement of the armature of motor E; .theswitch 25 being opened only when the damper /14 reaches its position forproducing a maximum cooling effect, and the limit switch 2S being openedonly when the damper 14 reaches a position for maximum delivery ofheat..

Temperature sensing bridge The temperature sensing bridge is composed ofa plurality of branches of two arms each. One arm of the bridge isprovided with a variable resistor which is not influenced by temperaturechanges to any appreciable extent, but all other arms of the bridge areprovided, in each case, with a highly sensitive temperature resistor.The number of bridge branches may be varied to accommodate the number oftemperature sensitive resistors desired to be used. However, for thepurpose of illustrations, the present embodiment is composed of threebranches. One branch comprises the arms 3i), 31; another branch includesarms 32, 33; and the third branch is composed of arms 34, 35. Theseveral arms 'constituting opposite sides of the bridge are connected atjunctions 36, 37, and these junctions are connected, by connectors 38,39, to a suitable source of power, and the junction 40 of the arms 30,31 is connected to the ground as indicated at 41. The arms 30, 32, 33,34 and 35 are provided with resistors 44, 4S, 46, 47 and 48,respectively, all of which have, preferably, a high negative temperatureco-eicient of resistance and the arms in which said resistors areinterposed have equal resistance. The resistors 45, 46 are enclosed insuitable housing a, 46a so as to hold heat within the housing for a`short period of time. An electrical heater F, having a predeterminedheating value, for example 2", is enclosed lin the housing with theresistor 45, and asecond electrical heater F' of like value is enclosedin the housing 46a with the resistor 46. These electrical heaters areconnected through energized closed contacts 49, 50 of said relay D, D'and are energized to effect momentary rebalance of the vbridge in themanner hereinafter described.

Transformer The source of power `for energizing the bridge and theelectronic elements of the system is a transformer comprising a primarycoil H and three secondary coils I, J and K, all of which are wound on acommon core. The primary coil H of the transformer has one end connectedto a main power line 51 and the other end is connected by conductors 52,53 to the ground 54. The conductor 53 serves also as a center tap forconnecting the center of the .transformer secondary winding I to theground at 54. The opposite ends ofthe secondary winding are connectedthrough windings `of relays D, D' to the electronic control elements.The bridge is energized by the secondary winding AK of the transformer;one end of this secondary being connected by ,said conductor 3S to thejunction 36 of the arms 30, 32 and 34, forming one side of the bridgeand the other end of secondary winding K is connected by said conductor39 to the junction 37 of the arms 31, 33 and 35 forming ,the other sideof the bridge. The secondary winding I serves as the source of energyfor the electrical heaters F, F'. In this connection, similar ends ofboth heaters F, F' are connected by common conductors 55 56, variableresistor 57 and conductor 58 to one end of the transformer secondarywinding J. The other end of this secondary winding is connected throughconductor 59, energized closed contact 50 of relay D and conductor tothe end 61 of electrical heater F. The same end of said vtransformerwinding J is connected by conductors 59, 62, closed `contact 49 of relayD and conductor 63 to the end 64 of electrical heater F', that whenrelay D is energized closed, the electrical heater F is energized toapply heat to the temperature sensitive resistor 45. Likewise, when therelay D is energized the electrical heater F is energized to apply heatto the temperature sensitive resistor 46. The heating value of bothelectrical heaters F and F may be varied by the adjustment of thevariable resistor 57. When the variable resistor 57 Ais adjusted to theposition shown in the drawing, keach electrical heater F., F will have amaximum heating value of 2. The temperature sensitive resistors 47 and48 are lpositioned in the -ar duct v10 so as to respond to thetemperature` of the air being delivered into the space B and thejunction 65 of the arms 34, 35 is connected through the connector 66 andvariable resistor 67 to the output point 42 of the bridge. Thetemperature sensitive resistor 48 is housed or otherwise shielded, asindicated at 68, from the direct influence of the temperature of saidair so that its response to the air temperature will lag relative to theresponse of the resistor 47. By reason ofthe lag response of theresistor 48 relative to the response of resistor 47, the resistor 47will promptly respond to any abrupt deflections of temperature in theair being delivered. Such abrupt deflection in the temperature ofdelivered air, if it is at or above a selected value determined by theadjustment of resistor 67, results in unbalancing the bridge and,therefore, produces acti vation of the electronic amplifier and therelay D or D', as the case may be, to adjust the position of the damper14 to compensate for the said abrupt l.temperature deflection. Theadjusted position of the variable resistor 67 eliminates the effect ofYall abrupt temperature deflections belowavalue of 5. I f

Referring again to ythe bridge branch composed of arms 30 and 31: Thearm 31has interposed therein the variable resistor 69 adapted to beadjusted to vary the resistance of arm 31 relative to arm 30. `Theresistor 69, as herein previously indicated, is not inuenced to anyappreciable extent by temperatures within the range dealt with hereinand, therefore, provides a simplified means for adjusting thetemperature setting of the bridge as awhole. When the resistor 69 isadjusted to establish a temperature setting for the bridge,"an`ydeflection of the temperature within the space B will produce aproportional variation in the resistance of temperature sensitiveresistor 44 in Such unbalanced condition of the bridge` results in avoltage appearing at the output point 42. This voltage is impressed byconductor 43 on a grid 70 of a vacuum tube V1, constituting a part ofthe electronic means G.

Electronic control means are both connected to ground at 73. The plate72V is connected through conductors 74, 75, and coupling condenser 76,to the grid 70a and'from there to the ground 54 through resistor 102,conductor 101, conductor 99, and resistor 100. The plate 72a isconnected, through conductors 77,Y 78, coupling condenser 79 andconductor 80, to one end of a variable resistor 81. The other end of theresistor 81 is connected to ground through a bias voltage supply as willbe explained later. The plate 72 Vthe bridge arm 30 and, therefore,unbalance the bridge.

is also connected, through conductors 74, 83 and negative feed backresistor 84, with the plate 72a.' The appropriate voltage for plate 72,72a is established by connecting them Ato both ends of the transformersecondary I. This connector includes rectifier branches 85, 86 leadingfrom opposite ends of said transformer secondary to a common conductor87, filter network comprising condensers 88, 89 and resistor 90,conductor 91 and resistor 92 to plate 72 and by said conductor 91 andresistor 93 to plate 72a. The slide 94 of variable resistor 81 isconnected by conductor 95 to the shield grid of each thyratron tube T1and T2. These tubes are of identical construction; The elements of tubeT1 comprise an indirectly heated cathode 95,.'a control grid 96, ashield grid 97, and a plate 98. The like elementsof the tube T2 aredesignated 95a, 96,

97"'and 98, respectively. The cathods 95, 95i`areco`nf nected to eachother by conductor 99 which is connected annesse through a resistor 100to the ground at 54. The control grids 96, 96L are connected to eachotherby conductor 103, the latter of which is connected to ground by aconnection comprising a resistor 104 and a condenser connected in shunttherewith. The plate 98 of tube T1 is connected by conductor 105 throughthe induction ,winding of relay D to one end of the transformersecondary winding I. The plate 98a of the tube T2 is connected byconductor 106 through relay D to the .other end of said transformersecondary winding I. A condenser 107 is connected in shunt with theinduction winding of relay D and a similar condenser 108 is connected inshunt with the induction winding of relay D. The filaments for heatingthe cathode of the tube V1 are designated 109, 110 and the filaments forheating the cathode of tubes T1 and T2 are designated 111 and 112. Allof the said filaments are connected in parallel relative to each otherin a circuit comprising conductor 58 connected to one endof thetransformer secondary winding I and conductor 113 connected to the otherend of said transformer secondary I. A full wave rectifier composed oftwo halves, 117 and 118 is connected across the conductors 58 and 113,connector 114 leading from the center tap of said rectifier throughdividing resistors 115, 116 to ground 119; which is also connected to acenter tap of transformer secondaryl winding I. Conductor 82 isconnected between voltage dividing resistors`115, 116 to pick up adirect current bias voltage which is impressed on the thyratron screengrids 97, 97a through variable resistor 81 and conductor 94. Condenser120 is connected in parallel with resistor 116 to eliminate anyalternating current voltage that may appear on conductor 82.

From the above it will be seen that the shield grids of thyratrons T1,T2 are biased to determine the amount of signal from plate 72a of tubeVlvwhich will cause the thyratron to begin'conducting, and that thisbias can be varied by adjustment of the slide wire on variable resistor81. i

It will also be observed that the thyratron tubes T1 and T2 are 180 outof phase by virtue of the connections of their respective plates 98 and98a to oppositeends of the transformer secondary I. Inasmuch as thetransformer secondary windings I, J and K are wound about a common core,the end 121 of the transformer secondary I to which the plate 98a' ofthyratron T2 and the relay D is connected, will be in phase with the end122 of the transformer secondary K connected to the junction 36 at oneside Vof the bridge. It would also be apparent that the end 123 of thetransformer secondaryI, to which the plate 98 of thyratron T1 and theinduction'winding of relay D-is connected, will be in phase with the`end 124 of `transformer secondary K which is connected to the junction37 at the other side of the bridge. Consequently, a voltage appearing atthe output point of the bridge in response to a decrease in theresistance of the temperature sensitive resistor 44 will result in`activating the thyratron T2 to energize and thereby close the relay D soas to eiect appropriateadjustments of the system to decrease the heatingeiects of the temperature altering device A. If the voltage appearing atthe output point 42 of the bridge results from an increase in theresistance of resistor 44 (which condition amounts, in effect, to aneffective decrease in resistance of resistor 69), the said voltage willbe in phase'with the thyratron T1 and will be impressed on its screengrid 97 and thereby effect energization of the relay D so as to effectappropriate adjustments of the system to increase the heating effect ofthe temperature altering device A.

The temperature sensitive resistors of the bridge network are highlysensitive to temperature changes. Consequently, the bridge will beunbalanced by very slight temperature deflections from the balancedsetting of the bridge as determined by the adjustment of the variableresistor 69. In order to avoid a performance of control operations inresponse to minor deilectionsof temperature the.. variable resistor 8,1,is interposed in the; connector. 8D between. the, coupling condenser 79.and thyratron grids 97 97a. While the said variable resistor 69 in thearm 3.1 of the.. bridge, is adjusted to; maintain balance of the bridgeat a selected temperature, the variable resistor 81 functionsv tocontrol theextent of unbalance in the bridge required to effect, controloperations of the system. In other words the resistor S1 may be adjustedto establish the upper and lower limits ot a temperature control band.For purpose of illustration, it may be assumed that the adjustedposition of the slide 94 on variable resistor 81, shown in the drawing,defines a temperature control band of 2.", that is to say, 1 deectionbelow the temperature setting ofv the bridge and lf above thetemperature setting of the bridge. However,y the spacing between theupper and lower limits ofthe temperature bandl may be increased bymovement of the slide 94 downwardlyto increase the effected resistanceof` conductor 80. lf the slide 94 is moyedin a direction to reduce theresistance in conductor 80, such adjustment serves to reduce the widthof the temperature control band.

Operation In connection with a description of the operation of thesystem, letit be assumed that the variable resistor 69 is so adjustedthat the bridge will balance at a temperature of 74 Within space B andthat the space is standing at thattempcrature, Let it be assumed alsothat the variable resistor 67 is adjusted to take into account abruptvariations of 5 in the temperature of the delivered air and that thevariable resistor 57 is adjusted to provide maximum heating effect of 2for each electrical heater F,I F and that the; resistor 81 is adjustedto provide a temperature control band of 2, that is to say, a bandhavingv an upper limit of 75 and a lower` limit of 73.

Under the above assumed conditions, there would be no controladjustments: of the system so long as the temperature of the airdelivered is maintained at a uniform rate and so long as the temperatureof the enclosed space stands between the. upper and lower limits of thetemperature. control band, between 73 and 75. However, it the spacetemperature should rise to 78, such temperature being above the upperlimit (75) of the temperature control band and also outside the 2heating effect of the electrical heater F, the voltage appearing at the.output point 42, as a result of this unbalanced condition of theVbridge, is` impressed by conductor 43 on the grid 70 ofthe. tube.V1 andthe amplified voltage is immediately-impressed on the grid 7l)a of saidtube V1 through conductor 7S and coupling condenser 76. inasmuch` as theinitial voltage appearing at the output 42 of the bridge, as a result ofan unbalanced condition due to decrease in the resistance of thenegative temperature co-efficient resistor`44, the original voltage andits amplification is in phase with the thyratron T2. Consequently, theamplicd voltage is impressed on the screen grid 97a of thyratron T2through they conductor 7S, coupling condenser 79, conductor 80,temperature band control resistor 81 and conductor 94. The function ofthyratron T2 energizes relay D through the conductor 106 which connectswith the; end 121 of transformer secondary I, the circuit continuingthrough the secondary I to the ground tap 53 and thence to, the cathode95 through resistor 160. and conductor 9.9., completing the circuit backto the plate 98.

The, energization or" relay D closes its contacts 22 and 5,0. The4closing of contact 22 directs an energizing circuit through the motor Eto adjust the damper 14 in a direction to reduce the volume of heatedair delivered and, therefore, increase the effectiveness of the coolair. Inasmuch as the temperature within the space B stands at 78, thisbeing 3 above the upper limit of the assumed temperature control band,they motor .E will continue. its operation until the temperature inspace B drops to 77, at which point cycling resistor 45 rebalances thebridge in the following manner:

'Jlhev Said. vheater F is energized. simultaneously withtheenergization4 o the motor E. by the closing of Contact 50. of relay D".AThis circuit leads from the end 125 of the transformer secondary l'through wire 58, variable resistor 57 and wires 56 andi 55' through. theheater F and thence through conductor 60, closed Contact 50 of relay Dand conductor 59 to the other endv 126 of. said transformer secondary l.However, the application. of the 2 of auxiliary heat to the temperaturesensitive resistor 45. is insutlcient to. rebalance the bridge while thetemperature of` the space stands above 77. However, when the temperaturewithin space B, is lowered to 77 the decreased resistanceof resistor 45,due to the 2 of auxiliary heat, applied,A is sufficient to otset theAsaid increased resistance in resistor 44 and thereby rebalances thcbridge. This rebalanccd condition of the bridgel is of short durationsince the rebalancing of the bridge results indeenergizing the relay Dand, therefore, opens the circuit through heater l?,y whereupon thebridge is again unbalanced as soon as. the effect of ther auxiliary heatis dissipated. This recurrent balancing and unbalancing of the bridge bythe application and removal of heat from the resistor 45. willcontinueuntil the temperature of the enclosed space B rises above theupper limit of the temperature control band. Inasmuch as the actuator ismoved by small increments each, time the bridge is unbalanced and isstopped when the bridge is balanced the damper will have assumed a newposition when the temperature of the space B reached 75 thereafter thedamper will maintain its last adjustment until the temperature of thespace B falls below the lower limit (73), of the temperature controlband. When this condition occurs, the bridge becomesl unbalanced becauseof an increase inthe resistance, of resistor 44,v thereby constituting,ineffect, a decrease inthe bridge arm 31. Therefore,v under suchconditions, the voltage appearing at the output 42 ofthe bridge will bein phase with the thyratron TI. Consequently,l this, voltage., whenamplified as previously described, is impressed on the screen grid 97 ofthyratron T1 and thereby produces energization of the relay D to close;its contacts 2 0 and 49'.

When the relayI contact 20 is energized closed it establishes a circuitthroughy motor E to impart movement to the damper 14 in a direction toopen the hot air duct 13 and therebyy increases the heating effect ofthe temperature altering device A., This circuit includes the mainsupply lead 19, energized closed contact 20 of relay D, conductor 27,motor field 24a', limit switch 28, and the motor armaturey to thegroundV 26. Simultaneously with the saidreverse energization of motor E,the closing of contact 49 of relay D closes an energizing circuitthrough heater F', whereby the auxiliary heat thus applied to thetemperature sensitive resistor 46 reduces its resistance. This heatercircuit leads; from the end of transformer secondary winding I throughconductor 58, variable rcsistor 57 and conductors 56 and 5,5 throughheater F and thencethrough conductor 63, energized closed contact 49 ofrelay D and conductors 62 and 59v to the end 126 of said transformersecondary J.

inasmuch as auxiliary heat is applied to the bridge resistor 46simultaneously with the reverse actuation of motor E assuming that thetemperature deflection above the lower limit of the temperature controlband, is less than 2-such reverse; actuation ot motor E will be onlymomentary and will, therefore, effectonly a slight adjustmentv of the.damper 14 in a direction to increase the heating effect of device A,since the rebalancing of the bridge. will de-energize the relay D and,therefore, open the heating circuit through auxiliary heater F,whereupon the bridge will become unbalanced by the dissipation of saidauxiliary heat'. This cycling action of the bridge will continue untilthe damper 14 is adjusted to a position to maintain the temperature ofthe enclosure B within the upper and lower limits of the temperaturecontrol' band.

arrechino Both limit switches 25 and 28 vremain closed so long as thedamper is in an intermediate position, the switch 25 being opened onlywhen the damper moves to a position to completely close the cool airduct 13 and the limit switch 28 being opened only when the damper movesto a position to completely close the hot air duct 12. Referring now tothe control function of the control device in response to abrupttemperature deections of the air within the duct The ,bridge arms34 and35, including the temperature sensitive resistance elements 47 and 48,are constructed to balance when both resistors 47 and 48 have attainedthe same temperatures. By reason of the shield 68 associated with theresistor 48, the responses of this resistor to abrupt changes intemperature will lag relative to the responses of the resistor 47.However, if the acceleration or deceleration of the air temperature induct10 persists for any appreciable period, the resistor 48 will attainthe same temperature as resistor 47, and, therefore, balance the bridge'arm 35 with relation to the bridge arm 34. The variableresistor67interposed in the connection-66 between the junction `65 of arms 34, 35and the output 42 of the bridge makes it possible to control the amountof temperature deflection necessary to unbalance the bridge. n

`When the arms 34, 35 are unbalanced as a result of abrupt decline inthe temperature of the air in duct 10, the resistance of resistor 47 isincreased with relation to the resistance of the resistor 48 and,therefore, sets into operation the means hereinbefore described foradjusting the damper 14 to offset said abrupt change in the temperatureof the air delivered. As a normal operation, the damper 14 will be movedto one or the other extreme positions in response to an abrupttemperature change withinY the duct 10. If the change results from anincrease in temperature the damper will close the hot air duct 12. Ifthe temperature change is a result of an abrupt decrease in temperaturethe damper 14 will be moved to a position to close the cool air duct 13.In either event, the auxiliary heat will be applied to one or the otherof the temperature sensitive resistors 45, 46, as previously described,so as to re-adjust the damper 14 to a proper position for maintainingthe temperature within the enclosure B within the upper and lower limitsof the temperature control band.

We claim:

l. A temperature control system comprising, in combination, atemperature sensing bridge composed of a plurality of arms of equalelectrical resistance, an electrical power source connected to oppositesides of the bridge, a temperature sensitive resistor interposed in oneof said arms and responsive to deflections in the temperature beingcontrolled to vary the resistance of said arm relative to the other armsof the bridge and thereby unbalance the bridge, a temperature alteringmeans including an actuator therefor for progressively adjusting thesame to counteract the temperature deflections in the temperature beingcontrolled, a temperature sensitive resistor interposed in another armof the bridge, an electrical heater for applying heat to the lastmentioned resistor to produce resistance variation therein to offset theeffect of the resistance variation in the first mentioned resistor andthereby momentarily rebalance the bridge, electrically energized circuitclosing means for energizing said heater and said actuator, and meansincluding an electronic switch connected in circuit with an output pointof the bridge and effective during an unbalanced condition in the bridgefor controlling the energization of said circuit closing means, wherebythe recurrent balanced and unbalanced condition of the bridge and theconsequent momentary energization and de-energization of said actuatorprogressively varies the effectiveness of the temperature altering meansto remove the said deection in the temperature being controlled.

2. A temperature control system as defined in claim 1 10 characterizedin that both saidresistors have a high nega.- tive temperaturecoefficient of resistance.

3. A temperature control system as defined in claim 2 characterized inthat both 'said resistors are interposed in arms at one side of thebridge, `whereby the heat applied to the second resistor producesresistance variations therein to counteract resistance variationsn thefirst mentioned resistor resulting from an increase in the temperaturebeing controlled. v

4. A temperature control system as defined in claim 2 characterized inthat the said resistors lare interposed in arms at opposite sides of thebridge, whereby the heat applied to the second mentioned resistorproduces resistance variations therein to counteract resistance"variations in the first mentioned resistor resulting from a decrease inthe temperature being controlled.

,5. A temperature control system comprising, in combination,` atemperature sensing bridge composedof'a plurality of arms of equalelectrical ,resistance defining opposite sides of the bridge, anelectrical power source connected across the bridge, 'a temperaturesensitive resistor having a high Ynegative temperature coeicient o fresistance interposed in an arm at one side of the bridge and responsiveto temperature deflections in the temperature being controlled to varythe resistance of said arm relative to the other'arms of the bridge andthereby unbalance the bridge, a temperature altering means including anactuator therefor for progressively adjusting the same tocounteract thetemperature deflections below and above a predetermined value of thetemperature being controlled, a second resistor having a high negativetemperature coecient of resistance interposed in another arm at the sameside of the bridge, a third such resistor interposed in an arm at theother side of the bridge, separately energized electrical heaters forapplying heat to the second and third resistors to produce resistancevariations therein to momentarily offset the resistance variationsproduced in said first resistor resulting from said temperaturedeflections, a first circuit closing means for the heater for applyingheat to the second resistor, thereby reducing its resistance to offset adecrease in the resistance of the first mentioned resistor as a resultof a temperature rise in the controlled temperature above saidpredetermined value, a second circuit closing means for energizing theheater for applying heat to the third resistor, thereby reducing itsresistance to otset an increase in the resistance of the first resistoras a result of a temperature drop in the controlled temperature belowsaid predetermined value, and means including an electronic switchenergized during an unbalanced condition in the bridge for selectivelyconnecting said circuit closing means with an output point of thebridge, whereby electrical heat is applied to one of said temperaturesensitive resistors to momentarily rebalance the bridge.

6. A temperature control system according to claim 5 characterized bythe provision of an electrically energized actuator operable in oppositedirections for adjusting said temperature altering means to increase anddecrease its effectiveness, one energizing circuit therefor beingconnected through the first circuit closing means and another energizingcircuit therefor being connected' through the second circuit closingmeans.

7. A temperature control system according to claim 6 in which the meansactivated by an unbalanced condition of the bridge to control saidcircuit closing means comprises an electronic network including avoltage amplifying tube and a pair of phase selector tubes, each beingconnected with said voltage amplifier and with one of said circuitclosing means.

8. A temperature control system according to claim 7 in which the saidphase selector tubes are connected in parallel with said voltageamplifying tube and out of phase with respect to each other, whereby oneselector tube is in phase with one side of the bridge and the 11 Qtherselector tube is in phase with the other side of the bridge.

9, A. temperature central. system according te claim 8 characterized bythe provisions of connectors leading from one of said circuit closingmeans to one end of a transformer secondary winding', the other circuitclosing means is electrically connected to the other end of the lastmentioned secondary winding, and in that the last mentioned secondarywinding and a similar winding vfor supplying power to the opposite sidesof the bridge are .wound on a Common corel0. A temperature controlsystem according to claim 5. characterized by the. provision of. avariable resistor interPQSed in an arrn of the bridge f or varying theresistancethereof to predetermine the temperature at which the bridgeWill rebalane.

11. A temperature control system according to claim 10l characterized bythe provision of means for selectively defining the upper and lowerlimits of a temperature control band within which band the movablecontrol elements et the system remain xed.

1 2, A temperature control system according to claim 11 characterized inthat means for selectively defining the upper andl lower limits of atemperature control band is a, variable resistor interposed in aconnection between the voltage amplifying tube and the selector tubes.

Y13. A temperature control system according to claim 11 characterized inthat the bridge includes a pair of arms, each being provided with aresistor having like temperature coefficients of resistance and exposedto the influence of a temperature medium, one of said pair of resistorsbeing shielded so as to lag in its response to abrupt temperaturechanges in said medium and thereby unbalance the bridge.

14. A temperature control system as defined in claim 13 characterized inthat a conductor leads from a junction between the, said pair of arms tosaid output point of the bridge and a variable resistor is interposed inthe last mentioned conductor to prevent minor changes in the temperaturemedia vfrom activating the amplifier tube.

References Cited in the le of this patent UNITED STATES PATENTS2,135,513 Holven Nov. 8, 1938 2,148,491 Moore. Feb. 28, 1939 2,246,575Coleman June 24, 1941 2,362,977 Crosthwait Nov. 21, 19,41 2,547,933Fitzgerald Apr. 10, 1951 2,574,925 Lehane Nov. 13, 1951 2,579,716 WilsonDec. 25, 1951 2,603,422 Sargeaunt July 15, 1952

